My wife's CPAP is a Phillips Dream Station 2. Its power requirements are 12V, 6.7A. The included 120V power supply is a 12V DC transformer which she uses at times when we are on shore power. We have a Phillips supplied adapter that we plug into 12V in the trailer so it runs off the trailer's battery when needed.
When going on a houseboat trip, I will pull our 100 amp hour lithium battery and take it with us along with the trailer's convertor to charge the battery after 2 night's use. Works great and is reliable. Can recharge in 2 hours off the boat's generator.